Ceramic cutting tool inserts are required to be hard, resistant to fracture and chipping, resistant to wear during high speed cutting and machining operations, resistant to oxidation and chemical corrosion or attack by the substance to be cut, resistant to heat and capable of dissipating heat generated by cutting and machining, and be relatively inexpensive to manufacture.
There are several types of ceramic cutting tool inserts which have the above desired properties in different degrees. One group of conventional ceramic cutting tool inserts are silicon nitride based. It is known to increase the hardness, thereby the wear resistance, and the fracture toughness of silicon nitride cutting inserts by adding densifying agents, such as various oxides, aluminum nitride and oxide, and refractory metal nitrides and carbides to the silicon nitride. It is commonly believed most of the above additives form glassy phases at the boundary of the silicon nitride particles, thereby leading to more dense products.
It is also known that the addition of nitrides such as titanium nitride, may further improve the performance of silicon nitride cutting tool inserts by providing solid state lubrication during high speed cutting of metals. Some conventional cutting tools are coated with a titanium nitride layer.
U.S. Pat. No. 4,900,700 issued to Jun et al. on Feb. 13, 1990, describes a ceramic composite containing silicon aluminum oxynitride with up to 10 weight percent titanium nitride. U.S. Pat. No. 5,250,477 issued to Baldoni II et. al. on Oct. 5, 1993, teaches a composite cutting tool and articles for similar use, made of silicon nitride, alumina and other oxides as densifying aids, additionally containing nitrides, carbides, borides and carbonitrides of transition metals in the form of particles and/or whiskers. Both the above patents specify broad composition ranges of the additives in amounts unrelated to one another.
It has been surprisingly found that desired properties of silicon nitride based ceramic cutting tool inserts may be notably improved if the additives for increasing hardness and density are present in interrelated amounts.